Saving data, especially e-mail and informal chats, is a liability.
It’s also a security risk: the risk of exposure. The exposure could be accidental. It could be the result of data theft, as happened to Sony. Or it could be the result of litigation. Whatever the reason, the best security against these eventualities is not to have the data in the first place.
via The importance of deleting old stuff—another lesson from the Sony attack | Ars Technica.